This invention relates to decoding techniques in which multiple instances of the same signal are transmitted over media having varying channel characteristics.
In a data transmission or storage system, it is desirable for information, often grouped into packets, to be accurately received at a destination. A transmitter at or near the source sends the information provided by the source via a signal or signal vector. A receiver at or near the destination processes the signal sent by the transmitter. The medium (e.g., a channel), or media, between the transmitter and receiver, through which the information is sent, may corrupt the signal such that the receiver is unable to correctly reconstruct the transmitted information. Therefore, given a transmission medium, sufficient reliability is obtained through careful design of the transmitter and/or receiver, and of their respective components.
However, the transmitter may be unaware of how the channel will affect a transmitted signal, and may not be able to transmit information in a way that will be effective for a particular channel. For example, the transmitter may be a wireless router, where the channel varies depending on its surroundings. One transmission technique to increase reliability when the transmitter does not have information about the channel is to increase the number of outputs (e.g., antennas) that the same information is transmitted. Thus, the same information may travel through multiple paths, allowing the receiver to more reliably estimate the transmitted information. This transmission technique is referred to as transmit or space diversity.
Even using transmit diversity, the receiver may still interpret the information incorrectly because of excessive channel effects and any additive noise that is independent of the channel. These errors may be corrected using an error correction scheme. To correct errors that may occur, redundancy is often added to a data stream. Therefore, when a reasonably small number of errors occur, there is still enough information to make an accurate determination of the transmitted sequence. The redundancy added to the data stream is determined based on an error correction code, such as a Reed-Solomon or Golay code.
Alamouti coding is one particular transmit diversity scheme that is used for wireless communications. In an Alamouti coding scheme at least two transmission antennas are used to transmit signal packets to one or more receiver antennas. The two antennas transmit a set of signal vectors during a first signal period and re-transmit the same (or equivalent) signal vectors during a succeeding signal period. At the receiver, an Alamouti decoder receives the two transmissions and combines them to recover the transmitted signal vectors. The Alamouti decoder recovers the signal vectors by reducing the effects of the transmission channel on the received signal. Typical Alamouti decoders assume that the characteristics of the transmission channel do not vary over time and, therefore, use the same channel estimates for processing the two successive transmissions. Typical Alamouti coding schemes therefore do not perform well when transmitted signals are coded over time and when the transmission channel (or characteristics of the transmission channel) varies over time.